This invention relates to the feedback of an X-Y positioning system, and particularly to a method and apparatus for improving precision of the position feedback over a limited area of the system.
For purposes of this disclosure, the following definitions apply:
Precision is defined as the smallest detectable position change. In an encoder, such a change is measured by the distance between pulses. In a vision system, we estimate that the smallest detectable distance is about half of a picture element (pixel) in a camera image. In a mechanical sensor gage, the precision is the least sensor movement which provides a detectable output change.
Accuracy is how close a desired position is to a true position, with measurement accuracy being the closeness of a measured quantity to the true quantity.
When picking up a component with the spindle of a pick and place head or the like and moving it in X and Y to a placement location at which the leads of the component are to be attached to corresponding conductive pads of a circuit board, it is often beneficial to transport the component to an inspection station intermediate the pick up and placement stations in order to determine the part's position and orientation relative to the spindle axis and then correct for any misalignment before placement. Often it is desirable to inspect the component also in order to determine whether all the lead positions and orientation are good compared to a nominal set of positions and orientations, and/or to measure the lead positions relative to each other very accurately.
In a typical case, it has been required to pick and place a four sided flat pack electrical component with correspondence between each lead and solder pad of 0.002-0.003 inches, wherein 0.014 inch wide leads extend from each of the sides at nominal center distances of 0.025 inches. A positioning system having a feedback encoder with a precision of 0.0005 inches is adequate for this purpose when no intermediate stop for inspection is necessary.
Additionally, a requirement for inspection of lead positions and orientations with a particular precision, say 0.0002 inches, can be performed at an intermediate inspection station using a camera 30 connected to a vision system 40 if both conditions are satisfied that the precision of the combined camera 30 and vision system 40 is equally or more precise than the particular 0.0002 inches, and that the whole component 2 (including leads 4) can be captured in one image of camera 30.
However with the present technology, the number of camera pixels in one image is limited for practical purposes, and several images may be necessary to inspect all the leads 4 of a component 2 with the required precision. Thus, the spindle 26 must be repositioned relative to the inspection camera 30 so as to reposition the component 2 and obtain successive different images of different portions of the component 2. Because of such repositioning, it is then necessary to know the position of spindle 26 relative to the camera 30 with a precision better than the required lead inspection precision so as to correctly determine the positions of the leads 4 in relation to each other.
From FIG. 4, it may be seen that the following vector assignments apply.
{X.sub.lead, Y.sub.lead } is the position of a reference point of a lead 4 relative to the axis of spindle 26.
{X.sub.cam1, Y.sub.cam1 } is the position of the optical axis of inspection camera 30 relative to machine zero.
{X.sub.leadcam1, Y.sub.leadcam1 } locates the lead relative to the optical axis of inspection camera 30.
By vector analysis, the lead position may be expressed as: EQU X.sub.lead =X.sub.cam1 +X.sub.leadcam1 -X.sub.enc ( 1) EQU Y.sub.lead =Y.sub.cam1 +Y.sub.leadcam1 -Y.sub.enc ( 2)
Thus, X.sub.lead, Y.sub.lead accuracy cannot be better than any of the component parts of equations (1) and (2), and
(i) X.sub.cam1 and Y.sub.cam1 are constant (and fixed relative to machine zero) and will not affect the relative measurement accuracy between leads. PA1 (ii) X.sub.leadcam1 and Y.sub.leadcam1 are known with the precision obtainable with the inspection camera 30/vision system 40. PA1 (iii) X.sub.enc and Y.sub.enc are known with the precision obtainable by the encoder.
In a particular case, a precision of 0.0002 inches is obtainable by camera 30/vision system 40, while the position information obtainable by encoder 22 of X-Y positioning system 20 is 0.0005 inches. Thus, the position of spindle 26 obtainable by encoder 22 is the least precise of the component parts (i)-(iii).
If we can inspect the leads with only one image of inspection camera 30, then encoder precision of 0.0005 inches is sufficient for positioning the component 2 at camera 30, since the encoder error will have the same effect on all of the leads relative to camera 30 when only inspecting the positions and orientations of all leads 4 relative to each other or to a nominal lead pattern.
The encoder error cannot be avoided when we are concerned with the positions and orientations of the leads 4 in X,Y, and .theta. relative to spindle 26, as in the situation where the spindle is repositioned to a placement location 8 for placement of the component 2 onto circuit board 6 with sufficient registration between each lead 4 and a solder pad 10. However, since the accuracy requirements for this registration between leads 4 and solder pads 10 is only 0.002-0.003 inches, the encoder error is acceptable.
As described above, the limitations of the camera 30 require that spindle 26 be repositioned relative to camera 30 for presentation of different leads 4 to the field of view of camera 30 so that several different images are required for inspection of the whole component 2. With such repositioning of spindle 26, the encoder error contained in X.sub.ENC and Y.sub.ENC cannot be avoided during the inspection process. Since the encoder precision of 0.0005 inches does not meet the required precision of 0.0002 inches for lead inspection, encoder 22 is inadequate for multi-image inspection of a component.
Accordingly, it is an object the invention is to provide a method and apparatus for overcoming this shortcoming while retaining the existing encoder.